Detect unreachable nodes for self-healing node pools

Detect unreachable nodes for self-healing node pools: a task in MiMo-V2.6-RL-oss: Agentic RL Environments (MiMo RL release). We're building a self-healing capability for node pools: when a managed node becomes unreachable for too long, the controller should delete it (so it can be recreated)…